I've had an iPad for a few months now, and absolutely love it - I use it for movies and music, games and browsing the internet. I've also done a few sketches and doodles using an app called Brushes. I've neglected to read the manual and preferred to get stuck straight in, but if you're used to Photoshop and layers, it's all pretty intuitive and incredibly enjoyable.
Here's one of the first 'finger painting' cartoons I did. To compensate for lack of pressure sensitivity, you can set it up to vary the line thickness or opacity with the speed you draw the line.
One of the neat things in Brushes is the ability to export the 'actions' into a quicktime movie. (Currently only available on Mac I think)
If you're thinking of getting an iPad, I recommend the Brushes app. I've yet to explore Sketchbook Pro in as much detail, but it looks equally good.